Mesh darkens after mirroring
This happens when you mirror manually instead of using the Mirror Modifier. There may be times when you want to mirror manually, but just be aware that this will happen and recalculate your normals afterward (by pressing CtrlN in Edit Mode with geometry selected). 

The reason this occurs is that each point is being moved to its mirror-opposite location and as it moves the outermost points actually cross over the innermost ones.

Let's take one step back. First you must understand what normal vectors, or "normals" are. They are the outward-facing directional vector that each face in your mesh has. If you want to see which way your normals are facing, you can enable this in your Mesh Display options.



Consider the analogy of a sock being turned inside-out. Here is our modeled sock, and as the duplicated sock is scaled on the X axis about the 3D Cursor, the vertices on the tips of the toes cross over the vertices on the rim of the sock, thus turning it inside-out.



And with face normals made visible you can see exactly what's happening. The brightly-colored prongs poking out from each face are a visual representation of your normals.



Vertices (and edges, sort of) have normals too, and you can display those also if you want, but they don't play a role in your shading the way face normals do. 

Should you be concerned about inverted normals? Yes. While they may not cause you trouble in all situations, there are plenty of situations in which they will - for example, they may cause your materials or textures to look wrong, cause texture baking to go wrong, cause your booleans to fail, cause 3D-printing to fail, and more.

The dark shading is Blender's viewport telling you: "I think this is the interior of your object", so if you see dark shading on the exterior you should examine your normals.

In short, you can prevent inverted normals by using a Mirror Modifier instead of manually mirroring, or simply recalculate normals afterward.